A sensor such as a nanomechanical sensor is well-known that includes a surface stress sensor (PTL 1, NPL 1) represented by a membrane-type surface stress sensor (MSS) and a cantilever sensor (NPL 2), and converts an item of target object into mechanical deformation or a stress of a sensing member so as to detect the obtained mechanical deformation or the stress by various means.
This type of sensor reads a surface stress or a change of weight induced by adsorbing a sample as deformation such as deflection and a change of resonant frequency respectively, to thereby detect the sample. The former is referred to as “static mode” and the latter is referred to as “dynamic mode”. FIG. 1 is a diagram conceptually illustrating the static mode and the dynamic mode as an example of a cantilever sensor.
In the static mode, a receptor layer on which the sample is adsorbed has a single coated surface in common. This is in order for the cantilever sensor or the like to be efficiently deformed due to the surface stress applied by adsorbing the sample. FIG. 2 is a diagram illustrating structures of the cantilever sensor (a), a both ends fixed beam sensor (b), the membrane-type surface stress sensor (c) and operations thereof in the static mode. FIG. 2 illustrates structures (a) to (c) which are formed of a micromachinable single crystal silicon Si (100) having a high piezoresistance coefficient, and a detection output with shade (resistance change: |ΔR/R|) obtained when the surface stress (3.0 N/m) is applied onto an area indicated by a light gray outline. In addition, FIG. 2 illustrates an enlarged top view of narrow portions in which piezoresistances are embedded.
Meanwhile, the above described “the piezoresistances are embedded” means piezoresistance portions are formed, and as described above, in a case where the sensor is formed of the single crystal silicon, a piezoresistive effect can be expressed on the aforementioned portion by doping an impurity such as boron only in a portion where the piezoresistance is to be formed. Such doping of the impurity can be realized by injecting ions on a required portion to be injected (specifically, a portion where stress is concentrated in FIG. 2) by an ion implantation method. The depth of ion injection is approximately 100 nm to 500 nm, and only the vicinity of a surface becomes the piezoresistance portion but the entirety does not become the piezoresistance in the depth direction. Meanwhile, as a matter of course, the piezoresistive effect cannot be expressed in a state where the ideal single crystal silicon has no carrier at all.
In a case of the single crystal silicon Si (100), since the detection output is given based on Expression of ΔR/R∝(σx−σy), in order to obtain a large detection output, the stress (σx≈σy, that is, ΔR/R≈0) uniformly applied by the surface stress needs to be converted into uniaxial stress (σx>>σy, or σ<<σy, that is, |ΔR/R|>>0) and amplified. In a common cantilever structure shown in FIG. 2(a), even though the width in the vicinity of the fixed end (“fixed portion” indicated by hatching) is made to be narrow, it is almost not possible to obtain the detection output (σx≈σy). FIG. 2(b) illustrates a both ends fixed beam structure. This structure is configured to fix both ends of the beam to which the surface stress is applied and has a simple shape with a good symmetric property to be relatively easily formed, thereby obtaining high sensitivity. FIG. 2(c) illustrates a membrane-type surface stress sensor (MSS) structure. The surface stress applied onto a center film can be efficiently detected as the uniaxial stress in which each of four narrow peripheral portions having the piezoresistance is amplified (two of right and left narrow portions of piezoresistance in total: σx>>σy, and two of upper and lower narrow portions of piezoresistance in total: σy>>σx, both cases lead to |ΔR/R|>>0). Accordingly, the highest sensitivity can be obtained. This structure has a highly symmetric property without free ends. Further, since four piezoresistances in total are connected with each other so as to form a full Wheatstone bridge (not illustrated), a stable operation is performed through self-compensation. In addition, in total of four piezoresistances in the upper, lower, right and left sides, the orientation of electric currents (for example, a [110] orientation of Si (100)) is aligned, and thus approximately four times output can be obtained.
Next, FIG. 3 conceptually illustrates a difference between deformations of a single-side-coated surface and a double-side-coated surface of the cantilever sensor which is operated in the static mode. In a case of the single-side-coated surface structure as shown in FIG. 3(a), the cantilever is deformed due to the surface stress on the single surface. Whereas, in a case of the double-side-coated surface structure as shown in FIG. 3(b), the surface stresses applied in both of the surfaces antagonize each other and thus the cantilever expands in a plane without being deflected. Therefore, since the cantilever is not substantially deformed, it is impossible to detect the sample based on the deformation. Particularly, when a detecting method of reading the cantilever sensor by means of optical methods such as a laser beam being employed in the static mode, it is impossible to detect the sample without deforming the cantilever in principle.
For this reason, the cantilever sensor has been required to employ “single-side-coated surface”, and thus various surface coating methods have been developed. Among the methods, an ink jet spotting method can be exemplified as a representative method (NPL 3). This is a method of coating only a single-side surface with a receptor layer by dropping a small amount of a solution of a receptor onto the cantilever by using an ink jet method used in a printer or the like.
However, there are problems in this method in that an ink jet nozzle is unstable due to the concentration and viscosity of a solution, a coffee ring effect can be found when the dropped solution is dried, and thereby it is difficult to coat the receptor layer having high quality with satisfactory reproducibility. In addition, since a process of normally immersing a substrate for several to several tens of hours is included to form a self-assembled monolayer which is important as a method of functionalizing a surface, it is impossible to form the self-assembled monolayer having high quality by using the ink jet method.
When the detection output is obtained by coating both surfaces instead of coating the “single surface”, it is possible to use, for example, a method of immersing all sensor elements in the solution of the receptor or a flow method of modifying a surface of sensor by allowing the solution of the receptor flow into the sensor element portion installed inside a sealed chamber. Accordingly, as long as a nanomechanical sensor of which both surfaces are coated with the receptor layer can be realized, it is possible to simply form the receptor layer having high quality with satisfactory reproducibility, which has been a problem of the nanomechanical sensor for a long time.
In an attempt to use the double-side-coated surface, the use of a piezoresistive cantilever has been reported (NPL 4). A sensor using the piezoresistance does not actually measure the “deflection” but a resistance change derived from the stress in accordance with the deformation such as the deflection. For this reason, in a case where both surfaces of the piezoresistance cantilever are coated by the receptor layer and the surface stress is applied to both of the surfaces, the stress generated by in-plane contraction/expansion is applied to the piezoresistance portion, and thus the detection output can be obtained.
However, in the cantilever structure, since the stress cannot be concentrated in the piezoresistance portion, the high sensitivity is not obtained. In addition, in a case where the single crystal Si(100) which has the high piezoresistance coefficient and can obtain the high sensitivity is used in a portion coated by the receptor layer or a portion, for example, where narrow regions are disposed in the vicinity of the aforementioned coated portion so as to concentrate the stress, there is a vital problem in that as illustrated in FIG. 2, the detection output can be rarely obtained by crystalline anisotropy in principle.